The present invention is in the field of photopolymerizable compositions useful as photoresists. In particular the invention deals with a laminated photopolymer composition with improved physical properties.
Photopolymerizable resist materials are known from U.S. Pat. No. 3,469,982 which describes a film resist in the form of a photopolymerizable layer sandwiched between a cover sheet and a temporary support. This film resist may be laminated to copper or copper-laminated board, exposed and processed to provide a photopolymerized layer which acts as a resist for subsequent etching.
A subsequent disclosure of manufacture of printed circuit boards using a laminated photosensitive layer is U.S. Pat. No. 3,547,730 which discloses the laminated layer achieving sufficient adhesion to the substrate to withstand subsequent photoresist processing steps of development, etching and plating.
Photoresists are well known in the art, and are commercially available under tradenames such as Riston.RTM. and Vacrel.RTM.. Photoresists typically comprise a cover sheet and a photopolymerizable layer which contains binder, monomer and photoinitiator. After the photopolymerizable layer is laminated to a substrate and exposed imagewise, the cover sheet can be removed and the layer is developed to produce an image. Some photoresist compositions are solvent developable and some are aqueous developable. The developed photopolymerized image serves to allow selective etching or soldering of the substrate.
A particularly demanding role for a photoresist film exists when it is used as a solder mask. The developed photopolymerized composition must be capable of resisting temperature of 300.degree. C.
The prior art teaches that solder masks may be prepared by coating, spraying or printing liquid compositions containing epoxy onto the substrate as in U.S. Pat. Nos. 4,237,216; 4,205,018; 4,171,977; 4,090,936 and 4,064,287. A method for preparing a film which can be laminated to a substrate is described in U.S. Pat. No. 4,029,348 which describes incorporation of an epoxy resin in a photopolymer layer along with a latent curing agent such as a boron trifluoride amine complex.
In U.S. Pat. No. 4,320,189 S-triazine compounds are disclosed for improving adhesion in photoresist lamination.
Thermal curing of the combination of an acid and a melamine compound is known as a means of obtaining superior mechanical properties in a coated film. In particular melamine compounds in combination with an acid functional unsaturated condensation binder and an ethylenically unsaturated compound are known from U.S. Pat. No. 4,247,621 for improving the mechanical strength of a photosensitive resin composition. This publication teaches that the binder comprise at least 50% unsaturated acid and have an acid value of 10 to 40. It is stated that unsatisfactory results are obtained with acid values below 10 or above 40. This publication further teaches that in addition to the acid functionality of the polyester that an optional acid such as p-toluene sulfonic acid can be added to accelerate the heat-hardening of the three essential components, i.e., (A) unsaturated polyester (B) photopolymerizable ethylenically unsaturated compound (C) melamine compound. The unsaturated polyester contains only terminal carboxylic groups to undergo crosslinking. The practice of this technique requires the balance of the amounts of (A), (B) and (C) to regulate viscosity and mechanical strength. This reference contains no suggestion of a means for producing a storage stable photoresist with a binder of higher acid value which can be laminated to a substrate.
An object of the present invention is to provide a photoresist film which is both highly stable at room temperature yet thermally curable at modest temperatures to improve physical properties.
A particular object of the present invention is to provide a solder mask element with improved hydrolytic stability.